High emissivity coating composition and method of use

ABSTRACT

A high emissivity coating composition for coating the interior of a furnace to direct thermal energy toward a load in the furnace wherein the furnace operates above 1100° C. thereby increasing the thermal efficiency of the furnace. The high emissivity coating composition includes a high emissivity agent and a binder agent. The preferred high emissivity agent is cerium oxide which defines a high emissivity factor from approximately 1000° C. to above 2000° C. The binder suspension agent is formulated to define the consistency and drying characteristics of paint such that the coating can be applied in a manner similar to the manner in which paint is applied. Moreover, the binder/suspension agent withstands the final use temperature.

This application is a continuation of U.S. patent application Ser. No.08/647,133, filed May 9, 1996, now U.S. Pat. No. 5,668,072.

TECHNICAL FIELD

This invention relates to the field of high emissivity ceramic coatingsfor improving the thermal efficiency of high temperature furnaces.

BACKGROUND ART

Ceramic coatings have been used for many years in a variety ofindustries. One particular use of ceramic coatings is in the field offurnace refractories. Specifically, ceramic coatings with highemissivity are applied to the interior of a furnace to improve theefficiency of the furnace.

Generally, high emissivity coatings are comprised of a refractorypigment, a high emissivity additive and a binder/suspension agent.Typical refractory pigments include zirconia, zirconia silicate,aluminum oxide, aluminum silicate, silicon oxide, etc. The highemissivity additive is typically a transition metal oxide such aschromium oxide (Cr₂ O₃), cobalt oxide (CoO_(x)), ferrous oxide (Fe₂ O₃),and nickel oxide (NiO). In some coatings, the refractory pigment and thehigh emissivity additive are the same material.

The binder/suspension agent allows the coating to be applied likeordinary housepaint and withstands the anticipated use temperature. Thebinder/suspension agent acts like a high temperature glue and istypically an aqueous solution or suspension of silicates or phosphates.

Several characteristics of a ceramic coating determine whether it can besuccessfully applied to a particular substrate. Typically, thermalexpansion matching, mechanical bonding, chemical bonding, and surfacestress characteristics have been considered as well as factors such ashigh temperature properties, corrosion resistance and wear resistance.Traditionally, the key concern for coatings to be applied in varianttemperature conditions was the match of thermal expansion coefficientsfor the coating and the substrate. The emphasis on thermal expansionteaches away from using materials with high coefficients of thermalexpansion. Cost is another factor which is considered when coatings areproduced. If the coating is more costly than the projected energysavings then the coating is not cost effective.

When a high emissivity coating is applied to the interior surface of afurnace, the thermal radiation properties of the refractory are enhancedthereby reducing fuel consumption and allowing increased productthroughput without increasing the average furnace temperature. A highemissivity coating allows thermal energy to be directed and redirectedtoward the furnace working zone and thereby best utilize the thermalenergy for the workpieces contained in the furnace. The coating reducesthe exterior temperature of the furnace since the thermal energy is keptwithin the hot zone and is not lost in the insulation and through thefurnace walls.

The heat treating industry has incorporated the use of high emissivitycoatings in furnaces. The heat treating industry typically operates attemperatures up to 1100° C. The coatings, which are typically chromiumoxide based, are effective and stable at temperatures under 1100° C.Unfortunately, at higher temperatures these coatings disintegrate over arelatively short period of time. Specifically, chromium oxide vaporizesat temperatures over 1100° C. Moreover, other standard transition metaloxide high emissivity additions also have high vaporizationcharacteristics as well as fluxing characteristics, whereby the entiremixture is lowered in overall melting point, preventing high temperatureuse for long periods of time.

The petrochemical industry is an example of an industry which utilizestemperatures up to 1650° C. (3000° F.) to process ethane, propane andsimilar hydrocarbons in a thermal-cracking process with steam to formethylene and propylene. It is desirable to use a high emissivity coatingin the thermal-cracking furnaces which can effectively withstand thehigher temperatures and can save fuel through better thermal efficiencyas well as evening out the hot zone (providing a more uniform furnacetemperature).

High emissivity coatings are also useful in the glass industry in largeglass furnaces. Moreover, high emissivity coatings are useful in largesteel melting furnaces.

Therefore, it is an object of the present invention to provide a highemissivity coating which can be utilized to increase the efficiency of afurnace at temperatures above 1100° C.

It is another object of the present invention to provide a highemissivity coating which is durable at temperatures above 1100° C.

Further, it is an object of the present invention to provide a highemissivity coating which has a consistency similar to that of housepaintand applied in the same manner as housepaint.

It is yet another object of the present invention to provide a highemissivity coating which is economical.

Moreover, it is another object of the present invention to provide ahigh emissivity coating which is durable in reactive atmospheres.

SUMMARY

Other objects and advantages will be accomplished by the presentinvention which provides a high emissivity coating composition which isdurable when applied to the interior of furnaces operating above 1100°C. The high emissivity coating composition of the present inventionincludes a high emissivity agent and a binder/suspension agent. The highemissivity agent is a rare earth based oxide wherein the rare earth iscerium or terbium. The preferred high emissivity agent is cerium oxidewhich defines a high emissivity from approximately 1000° C. to above2000° C. The binder suspension agent is formulated to define theconsistency and drying characteristics of paint such that the coatingcan be applied in a manner similar to the manner in which paint isapplied. Moreover, the binder/suspension agent withstands the final usetemperature.

DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention, a high emissivity coatingcomposition is provided with advantages over high emissivity coatings ofthe prior art. The high emissivity coating of the present invention isdesigned for use in furnaces for increasing the efficiency of thefurnace. Moreover, the coating of the preferred embodiment is economicalto produce and can be utilized for extended periods of time attemperatures above 1100° C. without degradation.

It is well known in the art that "blackbody" or high emissivity coatingsfor coating the interior of a furnace are desirable for improving theefficiency of the furnace. Further, it is desirable to have a coatingwhich is equally adherent to ceramic surfaces and metal surfaces. It isalso desirable to have a coating which exhibits durability, adequatehardness/abrasion resistance and stability in reactive environments.Moreover, it is desirable that the coating have an emissivityapproaching 1.0 in the operating temperature range.

The high emissivity coating is generally comprised of a refractorypigment, a high emissivity agent and a binder/suspension agent, asdiscussed above. Preferably, the coating defines a consistency such thatit can be applied like paint. The high emissivity agent in the coatingof the present invention is a rare earth based oxide wherein the rareearth is cerium or terbium.

Cerium oxide (CeO₂) is the preferred high emissivity agent. Cerium oxidehas a melting point of 2750K (2477° C.) and exhibits an emissivity ofapproximately 0.9 from 1000° C. to at least 2000° C. Cerium oxideexhibits an unusually high thermal expansion for a refractory material.Cerium oxide also exhibits a chemical resistance to both acids and basesand therefore cerium oxide can be utilized with acidic, neutral or basicbinder/suspension systems. Moreover, cerium oxide is economicallyfeasible to use in high emissivity coatings.

Cerium oxide is provided in powder form and typically has a particulatesize of about 3 to 15 micrometers, although finer and coarser particlesare occasionally available and are usable in the coating of the presentinvention. It will be noted that there are advantages of less shrinkageand better paintability if the particulate range is within the 3 to 15micrometers. Cerium oxide is typically available in 95 to 99.99 weightpercent pure. It will be noted that the present invention is not limitedto such grades of high purity cerium oxide. Mixed oxides of cerium oxide(i.e. cerium oxide-aluminum oxide, cerium oxide-silicon oxide) areusable as well as lesser pure forms of cerium oxide which contain otherrare earths. Moreover, precursors to cerium oxide such as hydratedcerium oxide (CeO₂ *H₂ O), cerium hydroxide [Ce(OH)₃ ], nitrates,acetates, hydroxynitrates, etc. are recognized as forming cerium oxideon heating to 500° C. and can be used instead of cerium oxide. Ceriumoxide typically refers to cerium (IV) oxide (cerium dioxide or CeO₂) butcan include cerium (III) oxide (cerium sesquioxide or Ce₂ O₃). Theanticipated maximum use-temperature of cerium dioxide (CeO₂) is 2000° C.and higher, whereas cerium (III) oxide (Ce₂ O₃) is about 1650° C.

In the high emissivity coating of the present invention, cerium oxideranges from approximately 5 weight percent to 75 weight percent in theliquid paintable coating. Although a higher weight percent is suitablefor use, the consistency of the liquid paintable coating becomespaste-like beyond 75 weight percent cerium oxide.

It will be noted that although the use of cerium is emphasized it is notintended to limit the present invention to this one rare earth. Terbiumis a rare earth which is similar to cerium and can also be utilized as ahigh emissivity agent in the present invention. A disadvantage to theuse of terbium is that it is expensive and is not economically feasiblefor use.

The preferred constituents and ratios of the binder/suspension agent are40-70 weight percent aluminum phosphate solution, 25-45 weight percentpeptized aluminum oxide monohydrate and 5-15 weight percent ethylalcohol. In the preferred embodiment, the binder/suspension agent iscomprised of 55 weight percent aluminum phosphate solution, 35.4 weightpercent peptized aluminum oxide monohydrate (PAOM) and 9.6 weightpercent ethyl alcohol.

In the preferred embodiment, the aluminum phosphate solution is acommercial binder Albrite "MALP" liquid manufactured by Albright &Wilson Company. The "MALP" liquid is generally comprised of 57 weightpercent monoaluminum phosphate trihydrate [Al(H₂ PO₄)₃ ], 2 weightpercent phosphoric acid, and 41 weight percent water.

The preferred form of the peptized aluminum oxide monohydrate isdescribed in U.S. Pat. No. 5,320,984, and is prepared by mixing 5 gramsof aluminum oxide monohydrate into 75 grams of water and adding 2.5 ofconcentrated nitric acid while stirring. This peptizing creates adispersed phase of ultrafine particulate material similar to colloidalmaterial.

In the preferred embodiment, the high emissivity liquid paintablecoating includes 51.6 weight percent cerium oxide powder and 48.4 weightpercent of the preferred binder/suspension agent. The 48.4 weightpercent liquid composition consists of 26.6 weight percent "MALP"liquid, 17.1 weight percent PAOM liquid, and 4.7 weight percent ethylalcohol. When mixed in these proportions, the coating has a consistencysimilar to that of paint such that the coating can be applied likepaint.

The "MALP" liquid has 41.7 weight percent nonvolatiles (solids formedwhen heating to 500° C.) and the PAOM liquid has 5.3 weight percentnonvolatiles. This paintable coating leads to an as-dried (at 500° C.)composition of: 81.1 weight percent cerium oxide, 4.8 weight percentaluminum oxide and 14.1 weight percent phosphorous oxide.

The binder/suspension agent discussed above defines a melting point ofapproximately 2050° C. Specifically, the phosphorus oxide vaporizes atapproximately 1650° C. but does not affect the aluminum oxide whichdefines a melting point of 2050° C.

It will be noted that although a preferred form of the binder/suspensionis disclosed above, the binder/suspension agent can includesolutions/suspensions based on a variety of other materials.Specifically, binders which are suitable for incorporating cerium oxideinclude low levels (0.5 to 3 weight percent) of cellulosic binders(sodium carboxymethylcellulose, hydroxypropylcellulose, etc.), orrefined bentonite or hectorite in a water medium. Also, mixed oxidebinders such as sodium silicate, potassium silicate, lithium silicatesolutions, and conventional colloidal suspensions or aqueous bindersolutions are suitable with the cerium oxide; these may, however, reducethe use-temperature to the lower end of the desired range. Hightemperature binders such as colloidal binders (namely colloidal silicaand colloidal alumina wherein colloidal refers to small particles,typically equal to or less than 0.05 micrometers, dispersed in water assols or hydrosols) or aqueous solutions of pure materials such asaluminum phosphate, aluminum nitrate, zirconium nitrate, zirconiumacetate, and yttrium nitrate may be used. Further, any combination ofthe binder/suspension agents as well as any acidic, neutral, or basicbinder suspension agent system which will achieve a finaluse-temperature in the range of 1000-2000° C. and is stable with thecerium oxide can be used. Inert liquids such as water, ethyl alcohol orothers can be used to develop the correct viscosity and dryingcharacteristics such that the coatings act like housepaint.

A binder/suspension system for higher purity includes ethyl alcohol,ethyl acetoacetate and cellulosic binder. When cerium oxide is the highemissivity agent, the preferred weight percentages are 18.6% ethylalcohol, 18.6% ethyl acetoacetate, 0.8% cellulosic binder and 62.0%cerium oxide. When dried to 500° C, the coating is comprised of 99.7weight percent cerium oxide and 0.3 weight percent carbon. When heatedabove 500° C., the carbon oxidizes leaving the coating with 99.9 weightpercent cerium oxide (assuming that 99.9% pure cerium oxide is used).With terbium oxide as the high emissivity agent, the preferred weightpercentages 17.9% ethyl alcohol, 17.9% ethyl acetoacetate, 0.8%cellulosic binder and 63.3% terbium oxide. When dried to 500° C., thecoating includes 99.7 weight percent terbium oxide and 0.3 weightpercent carbon. When the coating is dried above 500° C., the carbonoxides away and the coating is 99.9 weight percent terbium oxide(assuming that 99.9% pure terbium oxide is used).

An example on a non-aqueous paint, which upon drying leaves about 1%magnesium silicate clay, is comprised of ethyl alcohol, acetone,cellulosic binder, clay binder and a high emissivity agent. With 1weight percent magnesium silicate clay in the coating, there is notenough to create a large liquid phase such that the coating can beheated to the melting point of the high emissivity agent.

With cerium oxide serving as the high emissivity agent, the preferablepaint composition is 31.2 weight percent ethyl alcohol, 7.4 weightpercent acetone, 2.5 weight percent cellulosic binder, 0.8 weightpercent clay binder and 58.1 weight percent cerium oxide. When the paintis dried to 500° C., it is comprised of 98.1 weight percent ceriumoxide, 0.8 weight percent carbon and 1.0 weight percent magnesiumsilicate. When the coating is heated above 500° C. in air, the carbonoxidizes and yields 99.0 weight percent cerium oxide and 1.0 weightpercent magnesium silicate.

With the high emissivity agent being terbium oxide, the preferablecomposition of the paint is 30.2 weight percent ethyl alcohol, 7.1weight percent acetone, 2.4 weight percent cellulosic binder, 0.8 weightpercent clay binder and 59.5 weight percent terbium oxide. When dried to500° C., the coating is comprised of 97.9 weight percent terbium oxide,1.0 weight percent carbon and 1.1 weight percent magnesium silicate.When heated above 500° C. in air, the carbon oxidizes and yields 98.9weight percent terbium oxide and 1.1 weight percent magnesium silicate.

The preferred coating is created by mixing the cerium oxide which is inpowder form with the binder/suspension agent to create a"housepaint-like" coating. The coating is then applied like housepaint(brush, airspray, etc.) to the surfaces that will see the hightemperatures, typically the interior areas of high temperature furnacesand the furnace interior components. The coatings are generally 10-250microns thick. After the coating is dry to the touch, the furnace isready to use.

It will be noted that the coating of the preferred embodiment includesingredients that are volatile and must be outgassed away as the furnaceheats up. Moreover, the coating ingredients do not cause spalling of thecoating. Additionally, the inorganic ingredients of the coating act as ahigh-temperature glue, allowing the cerium oxide to bond to thesubstrate (ceramic or metal) tenaciously through a long use time,generally one to five years.

The binder/suspension agent of the preferred embodiment serves to bondthe cerium oxide to a ceramic substrate in a manner such that the highthermal expansion of cerium oxide is not a concern. The cerium oxidecoating breathes with the ceramic refractory to which it is applied. Ofcourse, a high thermal expansion is necessary when applying a ceramiccoating to a metal.

It will be noted that for inorganic binders the amount of the ceriumoxide must be such that there is at least 5 weight percent cerium oxidein the dried or sintered material. If only organic binders are used,typically only a low percentage of the organic is dissolved in water,such that the cerium oxide content of the dry paint will be high,possibly up to 99 weight percent; however, the cerium oxide must bond tothe substrate by high-temperature diffusion/reaction since there is noinorganic binder to aid in gluing the cerium oxide to the substrate.

The effectiveness of cerium oxide as a high emissivity agent fortemperatures above 1000° C. was illustrated via a comparative evaluationwhere three separate coatings were prepared and applied, in sections, tothe interior of a furnace. Each coating included the samebinder/suspension agent (the binder/suspension agent of the preferredembodiment). The only variable between each coating was the highemissivity agent which also served as the refractory pigment in eachcoating. The high emissivity agents were chromium oxide, silicon carbideand cerium oxide. After a year of furnace operation at 1400° C.-1500°C., the green chromium oxide had disappeared. Likewise, thegray-to-black silicon carbide had turned to a whitish, dusty coating.The cerium oxide coating appeared unchanged from when it was applied.

Moreover, further testing showed a 3 to 6% increase in the overallefficiency of the furnace with a marked reduction in the exteriortemperature of the furnace showing that less heat is lost through theinsulation and that the thermal energy is directed toward the loadwithin the furnace. Specifically, the product reached a higher averagetemperature in a coated furnace with the same thermal input. In thepetrochemical industry, fuel costs run in the hundreds of thousands ofdollars. A 3 to 6% increase in overall fuel efficiency can substantiallyreduce costs.

From the foregoing description, it will be recognized by those skilledin the art that a high emissivity coating and a method for use offeringadvantages over the prior art have been provided. Specifically, the highemissivity coating increases the efficiency of a furnace and is durablefor long periods of time in a reactive atmosphere at temperatures above1000° C. Further, the high emissivity coating defines a consistency anddrying characteristics similar to that of housepaint and applied in thesame manner as housepaint. Moreover, the high emissivity coating iseconomical.

While a preferred embodiment has been shown and described, it will beunderstood that it is not intended to limit the disclosure, but ratherit is intended to cover all modifications and alternate methods fallingwithin the spirit and the scope of the invention as defined in theappended claims.

Having thus described the aforementioned invention,

We claim:
 1. A method for improving the thermal efficiency of a furnace,said method comprising:applying a high emissivity coating to selectedsurfaces in the furnace, said high emissivity coating defining a coatingcomposition comprising a high emissivity agent and a binder agent, saidhigh emissivity agent being a rare earth oxide, wherein said rare earthis selected from the group consisting of cerium and terbium; drying saidhigh emissivity coating; and, heating said high emissivity coating to atemperature in the range of 1000° C. to 2000° C. thereby producing afinal coating, said final coating defining a high emissivity agentcontent ranging from 5 to 99 weight percent, said final coating beingbonded to said selected surfaces of said furnace, said final coatingbeing stable at temperatures greater than about 1000° C.
 2. The methodfor improving the thermal efficiency of a furnace of claim 1 whereinsaid coating composition is comprised of 5 to 75 weight percent of saidhigh emissivity agent and 25 to 95 weight percent of said binder agent.3. The method for improving the thermal efficiency of a furnace of claim1 wherein said high emissivity agent is cerium oxide.
 4. The method forimproving the thermal efficiency of a furnace of claim 1 wherein saidbinder agent defines a viscosity and drying characteristics similar topaint such that said high emissivity coating is applied by spraying orbrushing.
 5. The method for improving the thermal efficiency of afurnace of claim 1 wherein said high emissivity agent is cerium oxide,said coating composition being comprised of about 51.6 weight percentcerium oxide and about 48.4 weight percent of said binder agent.
 6. Themethod for improving the thermal efficiency of a furnace of claim 1wherein said coating composition contains 81.1 weight percent ceriumoxide when said coating is dried.
 7. The method for improving thethermal efficiency of a furnace of claim 1 wherein said high emissivitycoating defines a thickness ranging from 10 to 250 microns.
 8. Themethod for improving the thermal efficiency of a furnace of claim 1wherein said final coating is stable to at least 1650° C.
 9. The methodfor improving the thermal efficiency of a furnace of claim 1 whereinsaid high emissivity coating is capable of improving the thermalefficiency of a furnace which operates under standard operatingconditions for at least one year.
 10. A method for improving the thermalefficiency of a furnace, said method comprising:applying a highemissivity coating to selected surfaces in the furnace, said highemissivity coating defining a coating composition being comprised of 5to 75 weight percent of a high emissivity agent and 25 to 95 weightpercent of a binder agent, said high emissivity agent being a rare earthoxide wherein said rare earth is selected from a group consisting ofcerium and terbium; drying said high emissivity coating; and, heatingsaid high emissivity coating to a temperature in the range of 1000° C.to 2000° C. thereby producing a final coating, said final coatingdefining a high emissivity agent content ranging from 5 to 99 weightpercent, said final coating being bonded to said selected surfaces ofsaid furnace, said coating being stable at temperatures up to about2000° C.
 11. The method for improving the thermal efficiency of afurnace of claim 10 wherein said high emissivity agent is cerium oxide.12. The method for improving the thermal efficiency of a furnace ofclaim 10 wherein said binder agent defines a viscosity and dryingcharacteristics similar to paint such that said high emissivity coatingis applied by spraying or brushing.
 13. The method for improving thethermal efficiency of a furnace of claim 10 wherein said high emissivityagent is cerium oxide, said coating composition being comprised of about51.6 weight percent cerium oxide and about 48.4 weight percent of saidbinder agent.
 14. The method for improving the thermal efficiency of afurnace of claim 10 wherein said coating composition contains 81.1weight percent cerium oxide when said coating is dried.
 15. The methodfor improving the thermal efficiency of a furnace of claim 10 whereinsaid high emissivity coating defines a thickness ranging from 10 to 250microns.
 16. The method for improving the thermal efficiency of afurnace of claim 10 wherein said final coating is stable to at least1650° C.